Maintaining proper focus of objective optics can be critically important in high-resolution optical inspection systems, such as those used in production of semiconductor devices. Most inspection systems use auto-focus mechanisms based on optical sensing methods. Optical auto-focus sensing has the advantage that the quality of the optical focus can be sensed within the focal area of the optics without interfering substantially with the image capture function of the optics. For some applications, however, optical sensing may not provide sufficient focal precision, due, for example, to color variations on the inspected surface or to local height variations in the surface that are too small for the auto-focus system to track.
Other, non-optical types of non-contact position sensors are also known in the art. For example, Lion Precision, of Saint Paul, Minnesota, produces a line of capacitive sensors, which may be used in micro-positioning. These sensors are capable of making position readings with high bandwidth and tracking precision of 0.1 μm or less. A disadvantage of these sensors, however, is that they are optically opaque, and therefore cannot generally be used to make measurements in the focal area of an optical system without blocking at least part of the field of view of the optics.
The use of capacitive position sensors in automated focus adjustment of optical inspection systems has been described in the patent literature. For example, U.S. Patent Application Publication US 2002/0001403 A1, whose disclosure is incorporated herein by reference, describes a method for automatically focusing an ultraviolet objective lens, using a capacitance sensor near the objective lens. The capacitive sensor is used to measure a distance between the objective lens and an object under inspection, and the objective lens or object is moved based on the result of the measurement.
As another example, U.S. Patent Application Publication US 2002/0067477 A1, whose disclosure is incorporated herein by reference, describes the use of a capacitance-type sensor disposed near an objective lens to detect the distance between the objective lens and a semiconductor wafer under inspection. The focusing of the optical imaging system is adjusted by driving a moving stage vertically until the distance between the objective lens and the semiconductor wafer becomes optimal.